I have some models of three axis KPEV coach that have glasses behind the roof opening (Roco) and other (Fleischmann) that do not have (see yellow arrows).
Giving that, I suppose, these opening was intended for ventilation I wonder if glass reproduction is a mistake or not.
In the specific model in the above picture, ventilation is in charge to specific devices (Postdam or Grove or, after 1913/14, Wendler Sauger), present in spare part bag, to plug in the small square holes between the small windows.
So, in this case, it seems that glasses must be maintained.
Anyone can suggest me if I have to maintain glass reproduction or must I remove?
As far as I know, the special construction on the roof of prussian coaches was for better lighting of the compartments and not for ventilation. The construction is called in German "Oberlichtaufbau" ("skylight structure" in english?). The original coaches certainly had glazing in the places you meant. The Roco model is simply better than the Fleischmann model. Even better is the model from Brawa, where the parts of the roof between the glaces in the skylight structure are painted in the color of the coach, as it is true to the original.
In the late 19th century and early 20th century this construction principle was used by many types of coaches, e.g. Pullman cars, trams or for many Prussian coaches, like the compartment coaches or the D-Zug cars. Unfortunately there is nothing about using this construction principle for railway cars in the English Wikipedia page. Only in the German version these are mentioned: https://de.wikipedia.org/wiki/Laternendach The German term for this construction principle is "Laternendach" with the windows called "Oberlichter" as mentioned in previous comments. The windows had mainly the reason for additional lighting and the kerosene lamps in the cars were placed in this part of the roof. However, there were some cars using such windows for ventilation:
In the UK, it's a clerestory roof, primarily intended to provide better lighting.
Besides the Prussian State Railways who equipped each coach with clerestory roofs between approx. 1880 and its merger into the German State Railways in 1920, other Railways had clerestory roofs occasionally.
In the UK, in particular the Great Western and the Great Northern Railways had clerestory roof compartment coaches.
the caps on the roof (red arrow) are vents for the gas lighting. The tank underneath the wagon is the corresponding associated gas tank. The parts attached to the side of the row of windows on the roof (orange arrow) are the Wendler ventilators you already mentioned.
I must correct myself, the parts are lamp sockets, not vents. Normally, the corresponding gas pipes also run through the roof. As far as I know, when the gas lighting was replaced by electric lighting, the roof was simply smoothly closed. In my opinion, the gas tank under the vehicle also speaks against electric lighting. The unconventional shape of the lamp holders may also be an ÖBB modification or a simplification on the model; similar shapes, but with a kind of cap on the top, have also been implemented by Brawa, among others.
The coach may be gas-lit or Roco may have taken the epoch II prototype with gas lighting and simply transferred it into epoch IIIa ÖBB...
AFAIK, the additional parts of the Roco coaches also provide simple round, flat-topped parts to be mounted instead of the gas chimneys. When mounting these, the gas tank under the coach consequently needs to be replaced by accumulators.
I purchased two spare parts bags just for this purpose.
The are seven top flat caps exactly as the ones showed on the roof of the ÖBB coach.
Before seeing that model, I thought to simply eliminate the bell shaped device and substitute with the flat top plugs entirely plugged in to the roof, with function of seal of the previous chimney opening.
Than I see the above version and the doubt arose that they were not simple seals.
Spare bag also contains an electric current generator to substitute one of the tank (gas or kerosene one) to obtain an "electrified" version but, unfortunately, battery pack is absent.